Our understanding ofthe life cycles of EBV and HCMV has progressed so that we know much about their infections in tractable cell hosts including B-cells for EBV and fibroblasts for HCMV. We shall now focus on more technically demanding cell types central to the viral life cycles including epithelial cells for EBV and CD34+ cells for HCMV. EBV infects epithelial cells in vivo; it causes nasopharyngeal carcinomas, the undifferentiated form of which is uniformly EBV-positive. However, these tumors usually lose EBV upon explanting into cell culture. This loss may reflect EBV providing the tumor cells selective advantages that act only in vivo or for other unknown reasons. Conditions for infecting primary and established epithelial cells with EBV have recently been described, allowing a critical examination of EBV's life-cycle in them. We propose to characterize the dynamics of EBV visually in these tractable epithelial cells to elucidate how infections are established and maintained. In collaboration with Dr. Lambert in Project 1, we will assess the role of differentiation of epithelial cells in triggering EBV's lytic cycle. All of these experiments to visualize the life-cycle of EBV in epithelial cells will complement those to assess biochemically the latent to lytic switch in this cell type described in Project 4. We shall in parallel examine latent infections by HCMV, reactivation of which causes severe disease in the absence of a well-functioning immune system. At least one latent reservoir of HCMV is CD34+ hematopoietic progenitor cells. Primary CD34+ cells present experimental challenges such as heterogeneity and spontaneous differentiation, and thus our understanding of molecular mechanisms for the establishment, maintenance, and reactivation of HCMV latency is minimal. For example, unlike latent EBV, it is unclear if HCMV requires or even has mechanisms to replicate, maintain, and partition its genome within these cells. We have now infected established CD34+ cell lines and shown that they support latent infection of HCMV. We propose to characterize the dynamics of HCMV in these tractable CD34+ cells to elucidate how infections are established and maintained. Our collaborative studies of EBV and HCMV will use multiple approaches including live-cell imaging of visible derivatives of both viruses in these physiologically critical cell-types.